The presence of shaft currents in dynamoelectric machines (DEM's) has long been known, but is difficult to measure. Designers have postulated on methods of eliminating the troublesome circulating shaft currents which in some instances can rise to such levels that damage to the bearings, which journal the rotating shaft, often occurs.
Because of the difficulty in assessing the cause of bearing failure in operating DEM's, many bearings have been replaced in DEM's, the demise of which has often been attributed to bearing misalignment or lubrication failure when in reality, the cause of the bearing failure was caused by current circulation through the bearings of the machine and into the DEM pedestals where it could find a convenient return path.
In some troublesome machines, the useful life of a bearing set has been reduced to the order of hours or days by the presence of large circulating currents through the machine. The exact cause of the generation of shaft EMF's is not well understood, but is believed that the production of unbalanced magnetic fields in the DEM's result in generation of sizable zero sequence shaft voltages being induced in the DEM rotor shafts. The magnitude of the resulting circulatory currents in the shaft of the DEM is largely determined by the path resistance through the bearings and the ground return systems and not by the resistance of the shaft itself.
Many solutions have been attempted to ameliorate the circulating current problem and usually the most direct solution lay in increasing the resistance of the ground return paths.
The simplest solution utilized in the past, involved mounting part of the bearing structure in such a manner that the bearings themselves were insulated from their pedestals, or the pedestals were insulated from ground.